In general, a liquid crystal display device includes a liquid crystal cell formed by a pair of light-transmissive substrates placed to face each other and a liquid crystal layer formed between the light-transmissive substrates. Each light-transmissive substrate is provided with transparent electrodes arranged to form predetermined patterns, for applying a voltage to the liquid crystal layer.
The use of such a liquid crystal display device as the display of visual equipment such as television sets and information devices like computers is on the increase. Accordingly, there is a great demand for large-area displays recently, and developments of such a display have been actively carried out.
Liquid crystal display devices employing the STN (super-twisted nematic) mode and the TN (twisted nematic) mode have been known. In resent years, as a liquid crystal display device capable of achieving a display of higher definition and larger capacity, liquid crystal display devices using ferroelectric liquid crystals have been the focus of attention.
As taught by N. A. Clark and S. T. Lagerwall in Applied Physics Letters 36, pp. 899-901 (1980), ferroelectric liquid crystals have excellent characteristics, including a memory effect, high-speed response, and wide viewing angle. Moreover, ferroelectric liquid crystals achieve a high-definition, large-capacity display by a simple matrix system which is used in conventional TN-mode liquid crystal display devices and STN-mode liquid crystal display devices. The simple matrix system is used with a known structure where an electrode substrate having scanning electrodes formed by arranging transparent conductive films to form stripe patterns on a light-transmissive substrate and an electrode substrate having signal electrodes formed by producing stripe patterns using transparent conductive films on a light-transmissive substrate are disposed to face each other so that the scanning electrodes and the signal electrodes form a matrix pattern.
FIG. 18 is a sectional view showing the structure of a liquid crystal display element (liquid crystal cell) used in a conventional liquid crystal display device using a ferroelectric liquid crystal. As illustrated in FIG. 18, the conventional liquid crystal cell includes two pieces of glass substrates 171, 172 as opposed light-transmissive substrates. On a surface of the glass substrate 171, a plurality of transparent signal electrodes 173 made of a material like ITO (indium tin oxide) are arranged parallel to each other as transparent conductive films. Formed on the signal electrode 173 is a transparent insulating film 174 made of a material like silicone dioxide (SiO.sub.2).
On the other hand, on a surface of the glass substrate 172, a plurality of scanning electrodes 175 made of a material like ITO are placed parallel to each other as transparent conductive films so that the scanning electrodes 175 cross at right angles with the signal electrodes 173. The scanning electrode 175 is covered with an insulating film 176 formed from a material like SiO.sub.2. Alignment films 177, 178 which have undergone a uniaxial aligning treatment, for example, rubbing, are placed on the insulating films 174, 176, respectively.
The glass substrates 171, 172 are fastened together with a sealing material 180, and the space formed therebetween is filled with a ferroelectric liquid crystal 179. The ferroelectric liquid crystal 179 is injected through an inlet (not shown) formed in the sealing material 180, and the inlet is closed with a closing material 184 after the injection of the ferroelectric liquid crystal 179.
The glass substrates 171, 172 are sandwiched between polarizing plates 181, 182 which are positioned so that the polarization axes thereof cross each other at right angles. Moreover, spacers 183 are placed between the alignment films 177, 178, if necessary.
As illustrated in FIG. 19, a ferroelectric liquid crystal molecule 91 has a spontaneous polarization 92 in a direction orthogonal to the molecular long axis direction. The molecule 91 receives a force proportional to the vector product of the spontaneous polarization 92 and an electric field produced by a drive voltage that is applied across the signal electrode 173 and the scanning electrode 175, and moves on the surface of a conical locus 93.
Therefore, a viewer sees as if the molecule 91 switches between positions P.sub.a and P.sub.b of the axes of a liquid crystal locus as shown in FIG. 20. For example, if the polarizing plates 181, 182 are disposed so that their polarization axes coincide with the A-A' line and the B-B' line shown by the arrows in FIG. 20, respectively, a dark viewing field is obtained when the molecule 91 is in the position P.sub.a, and a bright viewing field is produced by double refraction when the molecule 91 is in the position P.sub.b.
The alignment states of the molecule 91 in the positions P.sub.a and P.sub.b are equivalent in elastic energy. Therefore, when the molecules 91 are aligned in a state, i.e., either the position P.sub.a or P.sub.b, by the application of an electric field, an optical state corresponding to the alignment state, i.e., a dark viewing field or a bright viewing field, is maintained even after the removal of the electric field. This is called a "memory effect". The memory effect is a unique characteristic of ferroelectric liquid crystal and is not associated with nematic liquid crystal.
Consequently, a display with higher definition and larger capacity can be provided by a simple matrix liquid crystal display device using ferroelectric liquid crystal having the memory effect and the high-speed response characteristic produced by the spontaneous polarization 92.
However, when adopting the ferroelectric liquid crystal into the simple matrix system, if a large-capacity, high-definition ferroelectric liquid crystal display device is fabricated by forming electrodes using only transparent conductive films of a material like ITO, the length of the electrode formed by the transparent conductive film becomes longer as the display area increases, resulting in a higher electrode resistance. Consequently, driving problems, such as generation of heat, delay of signals, rounding of the waveform of a signal applied to the pixel area, occur.
In short, the conventional TN liquid crystal display devices and STN liquid crystal display devices employ a multiplexing driving scheme in which a high-contrast display is obtained by scanning a plurality of frames with the periodical application of drive voltage. Therefore, degradation of display quality due to the delay of the applied voltage causes a little problem. However, in the case of a ferroelectric liquid crystal display device, it is necessary to form a high-contrast display by scanning one frame. Thus, the delay of the applied voltage would cause a problem.
For the reasons mentioned above, a large-area ferroelectric liquid crystal display device has been fabricated according to a method in which the overall electrode resistance is lowered by forming metal electrodes using a low-resistant metal film in a longitudinal direction of the scanning electrodes 175 and of the signal electrodes 173 made of transparent conductive films.
The requirements to be satisfied by the metal electrodes are that the metal electrodes are formed along the longitudinal direction of the transparent electrodes (the scanning electrodes 175 and the signal electrodes 173) which are formed by arranging the transparent conductive films to form stripe patterns, and that the metal electrodes are in contact with the transparent electrodes. The methods used for the formation of the metal electrodes are roughly classified into two types.
A first method is a method in which transparent electrodes are arranged to form stripe patterns on a transparent substrate (light-transmissive substrate), and metal electrodes are formed on the transparent electrodes so that the metal electrodes and the transparent electrodes are closely connected to each other. Three examples of the first method are as follows. (1) As shown in FIG. 21, a metal electrode 103 is formed on each of transparent electrodes 102, which are disposed to form stripe patterns on a light-transmissive substrate 101, along a side edge 102b of an upper surface 102a of the transparent electrode 102. (2) As shown in FIG. 22, a metal electrode 103 is formed on each of the transparent electrodes 102 so as to cover the side edge 102b of the upper surface 102a of the transparent electrode 102 and a side face 102c of the transparent electrode 102 (see Japanese Publication of Unexamined Patent Application No. 1-280724/1989). (3) As shown in FIG. 23, the transparent electrodes 102 are brought into contact with the metal electrodes 103 formed on an insulating film 104 coating the transparent electrodes 102, through long thin through-holes 105 produced in the insulating film 104 (see Japanese Publication of Unexamined Patent Application No. 1-280724/1989).
However, in the methods of (1) to (3), the metal electrode 103 protrudes from the upper surface 102a of the transparent electrode 102 by at least an amount equal to the thickness thereof.
When a ferroelectric liquid crystal element is used for a large-area panel, a necessary thickness of the metal electrode 103 as a low-resistant conductive film for reducing the delay of the applied voltage is preferably not less than 0.1 .mu.m, and more preferably not less than 0.4 .mu.m. Therefore, the thickness of the protruding portion of the metal electrode 103 from the upper surface 102a of the transparent electrode 102 needs to be at least 0.1 .mu.m, and increased with an increase in the area of the panel.
In order to realize a surface stabilized ferroelectric liquid crystal element, it is preferred to arrange the space between the facing electrode substrates to be about 1.0 .mu.m to 3 .mu.m. Hence, as the panel becomes larger in size, the possibility that the metal electrode 103 protruding from the upper surface 102a of the transparent electrode 102 comes into contact with the metal electrode 103 on the opposite electrode substrate increases.
Moreover, since the metal electrode 103 protrudes from the upper surface 102a of the transparent electrode 102, the surfaces of the insulating film and the alignment film covering the metal electrode 103 become uneven. As a result, the alignment of liquid crystal is disordered at the uneven surfaces, and the display characteristics lack uniformity.
In the second method, metal electrodes are arranged to form stripe patterns on a transparent substrate, and then transparent electrodes are formed on the metal electrodes so that the transparent electrodes and the metal electrodes are conductively in contact with each other. An example of the second method is as follows. As shown in FIG. 24, the metal electrodes 103 are arranged to form stripe patterns on the transparent substrate 101, and then the transparent electrodes 102 are disposed to form stripe patterns with the insulating film 104 between the metal electrodes 103 and the transparent electrodes 102 so that the metal electrodes 103 and the transparent electrodes 102 are in contact with each other through the through-holes 105 formed in the insulating film 104 (Japanese Publication of Unexamined Patent Application No. 63019/1990 (Tokukaihei 2-63019). In this method, since the thickness of the metal electrodes 103 can be increased compared to the first method, it is possible to further lower the electrode resistance.
However, the second method requires the processes of forming the insulating film 104 between the metal electrodes 103 and the transparent electrodes 102, and forming in the insulating film 104 the through-holes 105 for connecting the metal electrodes 103 and transparent electrodes 102.
Moreover, when the second method is adopted, the metal electrode 103 functions as a black matrix. In this case, regions A shown by cross hatching are covered with the metal electrodes 103 to prevent leakage of light from the spaces between adjacent transparent electrodes 102. Therefore, for the formation of the metal electrodes 103, it is necessary to consider pattern errors, and produce metal electrodes 103 wider than the regions A. As a result, overlapped sections where the transparent electrodes 102 overlap the metal electrodes 103 through the insulating film 104 are present. Therefore, although the insulating film 104 is present between the metal electrodes 103 and the transparent electrodes 102, the second method suffers from a problem that there is a strong possibility of a leakage current flowing between a metal electrode 103 and a transparent electrode 102 which is located next to a transparent electrode 102 corresponding to the metal electrode 103.
It would be possible to solve the drawbacks of the first and second methods by burying metal lines in the light-transmissive substrate. For example, Japanese Publication of Unexamined Patent Application No. 127494/1997 (U.S. patent application Ser. No. 08/744,171) discloses forming grooves on the light-transmissive substrate by etching, and burying conductors in the grooves to form a plane surface with the light-transmissive substrate. By burying metal lines as the conductors in the light-transmissive substrate, it is possible to overcome the above-mentioned drawbacks.
However, when burying the metal lines in the light-transmissive substrate using the technique disclosed in the Japanese Publication of Unexamined Patent Application No. 127494/1997, the thickness of the metal line is at most 1 .mu.m. In this case, the resistance is too high and the thickness of the metal lines is too small for the fabrication of large-area liquid crystal panel.
In either case, in order to drive a large liquid crystal display at high speeds, it is necessary to lower the electrode resistance so as to prevent a decrease of the voltage applied to liquid crystal in the pixel area. Therefore, the thickness of the metal electrodes 103 needs to made as thick as possible. However, when metal lines with a thickness of not less than 1 .mu.m are formed only by sputtering or vacuum evaporation, a peeled-off layer occurs and the metal surface becomes cloudy. Additionally, with regard to the deposition rate and cost, these methods are not suitable for practical applications.
In order to avoid the above-mentioned problems, it is necessary to add a process for increasing the film thickness, for example, plating. By employing a plating process, it is possible to increase the thickness of the metal lines. However, when the plating process is employed, it is extremely difficult to bury the metal lines in the light-transmissive substrate so as to form a plane surface with the light-transmissive substrate of Japanese Publication of Unexamined Patent Application No. 127494/1997. Meanwhile, in the structures shown in FIGS. 21 to 23, the metal electrodes 103 are formed on the transparent electrodes 102. In this case, when the thickness of the metal electrodes 103 is increased, it is more difficult to produce an electrode substrate with a flat surface.
Moreover, since the metal line is used as an auxiliary line of the transparent electrode, adhesion between the metal line and the light-transmissive substrate made of, for example, glass, and transparent electrode is required. Furthermore, for example, when other layer like a color-filter layer is provided on the light-transmissive substrate, adhesion between the metal line and the color-filter layer is required. However, when the metal line is made of a so-called low-resistant metal such as Cu (copper) and Al (aluminum), the adhesion between such a metal and the light-transmissive substrate and color-filter layer is not good. Therefore, a peeled-off layer that is a cause of a disconnection of the metal lines and a leakage current tends to occur at the contact sections of the metal lines and the light-transmissive substrate or the color-filter layer.
In addition, since the metal lines are exposed to various chemicals during etching and the development process of photoresist, the front face and side faces of the metal electrode 103 are readily oxidized or etched. This may cause conducting defects and disconnection of the metal lines. When the metal line is treated at high temperatures, the oxidation of the metal line further proceeds. Moreover, the low-resistant metals such as Cu and Al have extensibility, and are readily scratched in the rubbing process. Such scratches prevent good displays with uniform characteristics.
In order to achieve the above-mentioned structure where the metal lines are buried in the light-transmissive substrate, as illustrated in FIG. 25(a), a light-transmissive substrate 111 made of a material like glass and plastic is etched. However, since the light-transmissive substrate 111 does not have a crystalline structure, the light-transmissive substrate 111 is etched in an isotropic manner. Therefore, each of grooves 111a formed on the light-transmissive substrate 111 has a curved (tapered) surface on both sides thereof.
When depositing a metal in the grooves 111a, since a metal film grows according to the shape of the groove 111a, the resultant metal line 112 has protrusions 112a at the edges on both sides thereof as shown in FIG. 25(b). Namely, it is impossible to produce the metal lines 112 to form a flat surface with the light-transmissive substrate 111.
Furthermore, in this method, as the depth of the grooves 111a is increased to form the metal lines 112 with an increased thickness, the tapered faces of the grooves 111a become larger. As a result, the protrusions 112a also increase.
As a method of solving the problems of the first and second methods, a method proposed in J. Electrochem, Soc., 140, No. 8, August 1993, pp. 2410-2414 may be adopted. In this method, after patterning metal lines on a substrate, SiO.sub.2 is deposited on the substrate except the portions where the metal lines are formed by the LPD (liquid-phase deposition) technique so as to flattening the surface of the substrate.
However, in this method, although flat metal lines can be formed, the deposition rate of SiO.sub.2 is extremely low (20 nm/h). Therefore, it takes 100 hours to form, for example, a 2-.mu.m-thick film. In addition, since the film grows through a chemical reaction, the temperature of a solution and the concentration of each component of the solution affect the deposition rate. Hence, there is a need to strictly manage the solution used for the formation of the film.